This creates a custom window
//window @resize 100 200 300 400

This resizes it to be taller, 500 instead of 400

//window @resize 100 200 300 500

This instead resizes it to be slightly wider, width 350 instead of 300

//window @resize 100 200 300 400 | //window @resize 100 200 350 400

If you are wanting the script to change the size of the main mirc window itself, that is not possible. The
/help /showmirc
command can do some things to the main window, but not changing its size